THE INFLUENCE OF CYST NEMATODES AND DROUGHT ON POTATO GROWTH .2. EFFECTS ON PLANT WATER RELATIONS UNDER SEMI-CONTROLLED CONDITIONS

Potatoes were grown under a permanent rain shelter in mobile containers in soil with and without potato cyst nematodes (Globodera pallida). The plants were either subjected to an early drought stress period from planting until 43 days after planting, to a late drought stress period during tuber bulking or to a drought control. Leaf water potentials, stomatal diffusion resistances for water vapour, transpiration rates, dry matter accumulation and water use efficiencies of the plants were determined periodically. Both drought and nematodes decreased leaf water potential and increased stomatal resistance. Drought led to a higher water use efficiency, cyst nematode infection, however, reduced the water use efficiency at early stages of growth, and increased it at later stages. It is concluded that at least two main growth reducing mechanisms exist of which the relative importance varies with time. Firstly, reduced apparent assimilation rates, which are unrelated to a change in the water balance caused by the initial attack by the cyst nematodes. Secondly, a reduced dry matter accumulation resulting from a decrease of water uptake. Effects of drought and cyst nematode infection on plant growth and water relations were not always additive mainly because infected plants used up less water leading to less drought stress.